Hydrocarbon-based petroleum and synthetic oils derive from a variety of ultimate sources including crude oil, tar sands, shale oil, and liquefied coal-based compositions. Such oils are processed in refineries and chemical plants to remove undesired components and to chemically alter the hydrocarbon-based oils to manufacture streams having a higher value than the streams that either occur naturally or are delivered to processing equipment. Two such processes used in petroleum refineries are hydrotreating and hydrocracking.
A hydrotreating process typically reacts hydrogen, in the presence of a catalyst, with a hydrocarbon-based oil to convert organic sulfur and nitrogen compounds to hydrogen sulfide and ammonia, respectively, which can be relatively easily removed from the hydrocarbon-based oil stream. Various other reactions occur concurrently in the same reaction vessel including hydrogenation.
A hydrocracking process is similarly carried out in the presence of a catalyst, but typically at more severe conditions than used in hydrotreating. In particular, hydrocracking is typically carried out at a significantly higher pressure than hydrotreating and otherwise differs from hydrotreating in that an objective of hydrocracking is to break large molecules into smaller ones having a higher value.
Hydrogen is used in both processes, and since the processing units are operated at relatively high pressures, the capital and operating costs for compression are significant. Various inventions have been disclosed pertaining to the configuration of the processing units with respect to the hydrogen system, frequently with the objective of lowering capital and operating costs, while increasing the flexibility of the processing equipment.
U.S. Pat. No. 3,592,757 issued to Baral teaches a hydrofiner (essentially the same as a hydrotreater) operating in series with a hydrocracker, with a fraction of the product fed to a hydrogenator. A gas oil feed is fed with both make-up and recycle hydrogen to a hydrofiner. A recycle stream and additional recycle hydrogen are added to the hydrofiner product stream, and the mixture is fed to a hydrocracker. The hydrocracker product stream is cooled and separated into a vapor and a liquid stream. The vapor stream is passed to a recycle hydrogen compressor for recycle to the hydrofiner. The liquid stream is fractionated into top, mid, and bottom streams. The bottom stream is recycled to the hydrocracker. The mid stream is mixed with hydrogen from a make-up hydrogen compressor and directed to a hydrogenator. Hydrogen recovered from the hydrogenator is compressed in a stage of the make-up hydrogen compressor and directed to the hydrofiner.
U.S. Pat. No. 5,114,562 issued to Haun et al. teaches a two-stage hydrodesulfurization (essentially the same as a hydrotreatment) and hydrogenation process for distillate hydrocarbons. Two separate reaction zones are employed in series, a first for hydrodesulfurization and a second for hydrogenation. A feed is mixed with a recycled hydrogen and fed to a desulfurization reactor. Hydrogen sulfide is stripped from the desulfurization reactor product by a countercurrent flow of hydrogen. The liquid product stream from this stripping operation is mixed with relatively clean recycled hydrogen and the mixture fed to a hydrogenation reaction zone. Hydrogen is recovered from the hydrogenation reactor and recycled as a split stream to both the desulfurization reactor and the hydrogenation reactor. The hydrogen from the stripping operation is passed through a separator, mixed with the portion of the recycled hydrogen directed to the hydrogenation reactor, compressed, passed through a treating step, and recycled to the hydrogenation reactor. Thus, the hydrocarbon feed stream passes in series through the desulfurization and hydrogenation reactors, while relatively low pressure hydrogen is provided for the desulfurization step and relatively high pressure hydrogen is provided for the hydrogenation step.
U.S. Pat. No. 5,403,469 issued to Vauk et al. teaches a process for producing fluid catalytic cracking unit (FCCU) feed and middle distillate. Separate feed streams from a vacuum tower are processed in parallel by a hydrocracker and a hydrotreater, a relatively lighter feed stream in the hydrocracker and a relatively heavier feed stream in the hydrotreater. A common source of recycled and make-up hydrogen is fed in parallel to the hydrocracking and hydrotreating steps. The product streams from the hydrocracking and hydrotreating steps are separated into liquid and vapor streams in a common separator. Consequently, the hydrocracking and hydrotreating steps operate at the same pressure. This requires either the hydrotreating step to operate at a higher than optimum pressure and/or the hydrocracking step to operate at a lower than optimum pressure, since typically a hydrocracker is operated at a significantly higher pressure than a hydrotreater. With make-up hydrogen added to maintain pressure, recycle hydrogen is recycled from the common separator to a recycle gas compressor, which compresses the gas before parallel delivery to both the hydrocracker and the hydrotreater. In an alternative embodiment, the feed to the hydrocracker is a recycle stream from a fractionator that separates the combined product from the hydrotreater and the hydrocracker.
Although there have been many advances in this art, there remains a need for a parallel hydroprocessing configuration, where parallel reactors operate at different hydrogen partial pressures, but yet, capital and utility costs for compression are reduced relative to conventional configurations.